1. Field of the Invention
The invention relates to a tilt servo apparatus and a control method for the apparatus and, more particularly, to a tilt servo apparatus which is used in a recording/reproducing apparatus of an optical disc and a control method for such an apparatus.
2. Description of the Related Art
In a recording/reproducing apparatus of an optical disc such as CD (Compact Disc), DVD (Digital Versatile Disc), or the like, a deviation occurs between an optical axis of a laser beam irradiated from an optical pickup and a direction normal to the optical disc at the beam irradiating position due to a warp or the like of the optical disc. An angle of such deviation is called a tilt angle, and occurs mainly in the direction of radius of the optical disc (hereinafter, referred to as xe2x80x9cradial directionxe2x80x9d), and which causes a coma-aberration or the like of an optical system. When the tilt angle occurs, signal deterioration such as crosstalk to an adjacent track, jitter, or the like appears which exerts an adverse influence on the reproducing quality of the optical disc. Since it is necessary to shorten a wavelength of the laser beam and to increase a numerical aperture NA of an objective lens in order to reduce a spot diameter of the laser beam, particularly, in such case as to perform a high density recording such as in a DVD system, a margin for the tilt angle decreases. Thus, the reproducing quality is greatly deteriorated even if the optical disc is slightly inclined. To correct an aberration due to the tilt angle during the reproduction of the optical disc, therefore, a disc player is generally provided with a tilt servo mechanism for correcting the tilt error on the basis of a detection signal intensity of the reflected laser beam.
It is, however, required that the tilt servo mechanism operates at a high speed and, particularly, in a reproducing apparatus which handles an optical disc of a high density recording, a tilt servo mechanism is necessary which performs a high speed operation. In a tilt servo apparatus using a liquid crystal device in order to correct an aberration, there is such a problem that a response speed of the liquid crystal decreases due to a reduction of an environmental temperature and the operation of the tilt servo becomes slow.
The present invention is made in consideration of the problems and it is an object of the invention to provide a tilt servo apparatus which operates at a high speed and with high precision irrespective of a magnitude of a tilt error and a change in environmental temperature.
A tilt servo apparatus according to the present invention comprises: photodetecting means for detecting a reflected light beam of a laser beam irradiated to an optical recording medium, tilt error correcting means for correcting a tilt error occurring due to a tilt angle which is formed between an optical axis of the laser beam and a direction normal to the optical recording medium at the laser beam irradiating position, control means for controlling the tilt error correcting means on the basis of a detection signal intensity of the photodetecting means, correction value changing means for changing a correction value of the tilt error correcting means by a first correction width; comparing means for comparing an amount of change of the detection signal intensity when the correction value is changed by the first correction width with a predetermined amount of change; and correction width changing means for changing the first correction width on the basis of the comparison result of the comparing means.
According to another aspect of the present invention, if it is determined by the comparing means that the amount of change is smaller than the predetermined amount of change, the correction width changing means changes the first correction width to a second correction width smaller than the first correction width.
According to further aspect of the present invention, the comparing means also discriminates a reduction of the detection signal intensity when the correction value is changed, and when the reduction of the detection signal intensity is determined, the correction width changing means reverses an increasing/decreasing direction of the correction value and calculates the correction width.
According to further another aspect of the present invention, the tilt error correcting means is a liquid crystal device for aberration correction arranged on the optical axis of the laser beam, and the control signal of the control means is a driving signal of the liquid crystal device.
According to further aspect of the present invention, the apparatus further comprises temperature detecting means for detecting a temperature of an environment where the liquid crystal device is arranged, wherein the correction width is determined on the basis of the detected environmental temperature.
According to further another aspect of the present invention, the apparatus further comprises storing means for storing the correction value in which the amount of change is smaller than a predetermined threshold value as an optimum value of the tilt error correction, wherein the control means uses the stored optimum value as an initial value of the correction value of the tilt error correcting means.
According to further aspect of the present invention, the apparatus further comprises level changing means for changing a level of the detection signal intensity so that the detection signal intensity does not exceed a predetermined upper limit value.
A tilt servo apparatus according to the present invention comprises: photodetecting means for detecting a reflected light beam of a laser beam irradiated to an optical recording medium, tilt error correcting means for correcting a tilt error occurring due to a tilt angle which is formed between an optical axis of the laser beam and a direction normal to the optical recording medium at the laser beam irradiating position, control means for controlling the tilt error correcting means on the basis of a detection signal intensity of the photodetecting means, discriminating means for discriminating an increase/decrease and an amount of change of the detection signal intensity when a correction value of the tilt error correcting means is changed by a predetermined value; calculating means for calculating a correction width on the basis of a discrimination result of the discriminating means; and determining means for determining a plurality of correction values which are different from one another by the calculated correction width. The control means controls the tilt error correcting means so as to obtain a correction value in which the detection signal intensity becomes the maximum among the plurality of correction values.